Wire wound resistor terminal cap



Sept- 12, 1961 L. w. BERNI 2,999,996

WIRE woUND RESISTOR TERMINAL CAP Filed Jan, 16, 1961 www ATTORNEYS United States Patent setts Filed Jan. 16, 1961, Ser. No. 83,109 8 Claims. (Cl. 338-329) The present invention relates to wire Wound resistors, and particularly to terminal caps for such resistors, and 1s a continuation-in-part of my copending application, Serial No. 836,154, filed August 26, 1959, and now abandoned.

Wire wound resistors in general comprise an elongate base member of electrical insulation material which functions as a structural support or mandrel upon which a desired length of resistance wire is helically wound. Such units are usually employed as the resistance elements in potentiometers, rheostats, or variable resistors in general. The elongate base member may be curved or straight, depending upon whether a rotary, or rectilinear potentiometer is desired, land it may be uniform in transverse dimension or non-uniform, depending upon whether an electrically linear or non-linear, regular or irregular, resistance function is desired. Regardless of the shape of the resistance element, provision must be made for effecting one or more fixed electrical lead connections to the resistance element, in order that it may be electrically incorporated into a circuit. The present invention is concerned with the provision of such terminals.

In the lield of precision miniature variable resistors, the resistance Wire used to form the helically wound resistance element is usually extremely line and delicate. Consequently, if one provides a terminal lead merely by soldering or welding the ends of the resistance wire to a conductor lead, such junction is not only diicult to effect, but because of the lineness of the single strand of resistance wire, it is mechanically weak and uncertain. Also, if one provides a terminal cap such as by welding or solder-ing a metal foil member over the top of a portion of the line wound element: when enough heat is used to obtain a good bond with the wire strands, frequently the wire strands are melted because of their fineness and a mechanically Weak and electrically noisy junction is obtained; and on the other hand, when a lesser degree of heat is used, the bond is frequently mechanically weak. The delicate balance between enough heat for a good bond and not so much heat as to melt the line strands of resistance wire, is difficult to obtain, and even with the best skill non-uniform results are obtained.

The present invention overcomes the foregoing disadvantages, and provides terminal caps for wire wound resistance elements, which are uniformly strong and electrically sound, and which may be readily fabricated even on very fine resistance wire without an undue requirement of skill on the part of the operator. In general, the terminal caps of the present invention are provided on wire wound resistors by sandwiching a section of the resistance wire convolutions between two thin plate or foil members, one plate between the wire and its insulation base, and the other plate over the wire. These two plates are then welded together. The materials of the two plates are chosen to have a lower melting point than the resistance wire; and at least one of these materials is chosen to have a llow point below the melting point of the wire, so that in the welding process one of the weld metals ows over and around the strands of resistance wire, encasing the strands, and forming a rugged, reliable, and electrically sound junction ywith the wire. 'Ihe plate positioned over the wire convolutions Mce is preferably provided with an extending tab to which conductor leads may be readily soldered or welded.

The welded sandwich or laminated structure itself does of course distribute the mechanical forces over the entire area of the sect-ion of resistance wire embraced within the sandwich, thus practically eliminating the possibility of localized strain on single strands of resistance wire, with the resultant likelihood of breakage. In addition, the flow of weld metal to encase the resistance wire strands further strengthens and improves the structure by reinforcing the strands and obtaining a secure and unvarying electrical connection between the resistance wire land the terminal plates.

It is accordingly one object of the present invention to provide connection terminals for wire wound resistance elements.

vide connection terminals for wire wound resistance elements, wherein the resistance wire portion forming the terminal is reinforced by lamination between two terminal plates, and is embedded within the material of at least one of the plates.

lOther objects and advantages of the present invention will become apparent to those skilled in the art from a consideration of the following detailed description of one exemplary embodiment thereof, had in conjunction with the accompanying drawings, where certain details have been exaggerated for purposes of illustration, and in which like numerals refer to like or corresponding parts, and wherein:

FIG. l is a vertical longitudinal section through a rectilinear insulation strip having a resistance wire wound thereon, and several terminals partially assembled therewith;

FIG. 2 is ya top plan view of the strip of FIG. l, with the assembly of the terminals completed;

FIG. 3 is an enlarged longitudinal sectional view of a completed terminal assembly, taken along line 3-3 of FIG. 2; and

FIG. 4 is an isometric view of a completed wire wound resistance unit severed from the str-ip of FIG. 2, having a terminal assembly at each end thereof.

Wire wound resistance elements for use in variable resistor, rheostat, and potentiometer devices, particularly of the miniature variety, are usually in the form of a helically wound resistance wire applied over an insulation base or card as the mandrel for the winding. In the specific example of the drawings, the insulation base or mandrel 10, which may be a non-conducting plastic, fiber board, plastic-fiber laminate, or the like, is shown in the form of a thin, Hat, narrow strip of uniform crosssectional dimensions. The strip 10` may be of indefinite length, if desired, and the resistance wire 11, which may conventionally be a Nichrome resistance wire., is continuously helically wound thereon to produce what eventually will become a plurality of individual resistance units.

In accordance with the present invention, the card 10 and winding 11 are delineated into a plurality of resistance units by the terminal assemblies comprising elements 12 and 15 spaced at intervals along the length `of strip 10. As is apparent from FIG. l, the elements 12 underlie the sections 11a of the wire winding 11. This is accomplished by placing the elements 12 on the card or strip 10 in advance of the application of the winding 1 1 to such locality. Since it is conventional to varnish the card 10 before application of the wire winding 11, and to apply the winding while the varnish is tacky, this tacky varnish coating functions to hold the elements 12 in position until the winding has been applied thereover. After the Winding 11 and elements 12 have been applied to the strip 10, elements are superposed on the areas 11a of the winding over the Wire strands and the elements 12, and as will be described in greater detail subsequently, the elements 15 are resistance welded to the underlying resistance wire 11 and elements 12, sandwiching the winding sections 11a therebetween. Strip 1i) can then be out into discreet resistance elements by severing through the terminal assemblies 12, 15, to provide units such as illustrated in FIG. 4, having a terminal at each end.

Considering the terminal assemblies lin greater detail, the underlying element 12 of the assembly is a small rectangular platelet comprised of two laminae, a base 14 of brass, e.g., with a layer 13 of a conventional silver solder or the like fused thereto. The superposed element 15 is a small rectangular platelet. When Nichrome wire is used for the resistance element 11, it is preferred to use nickel for the platelet 15. Platelet 15 is chosen to be nickel in order to be similar in expansion properties to the Nichrome Wire 11 adjacent thereto, and particularly to approximate the melting point of the wire.

In place of conventional silver solder as mentioned above, one may use any other suitable high temperature solder, Vand the choice of a suitable solder will of course be dictated by the particular materials used for the platelets and for the resistance wire as will be readily understood by those skilled in the art, bearing in mind that the melting point of the solder employed should be approximately 50% to 75% less than the melting points of any of the materials joined by the series resistance welding operation. A number of suitable high temperature solders are listed:

Solder Composition: Melting Point, F.

Ag 35%, Cu 26%, Zn 21%, land Cd 18% (common silver solder) 1125 Ag 54%, Cu 40%, Zn 5%, and Ni 1% 1340 Ag 9%, Cu 53%, and Zn 38% 1410 Ag 95%, and In 5% 1688 Au 99%, and Al 1% 1652 Au 99%, and As 1% 1229 In forming an integrated terminal, when the elements are in position as shown in FIG. l, a pair of resistance welding electrodes are brought to bear on the-upper surface of platelet 15 in a plurality of locations, such as indicated by the pairs of weld spots 16 shown in IFIG. 2 The result of the heat generated by the resistance welding electrodes is illustrated in FIG. 3. The heat welds the platelet 15 to the wire 11 in the areas around the application of the electrodes to the platelet, and melts the silver solder layer 13, causing it to flow up and around the wire strands 11a and fuse or alloy with the nickel top platelet 15. Thus the strands of resistance wire in the area 11a are sandwiched between the platelets 12 and 15, and are embedded in the solder lamina 13, while said solder lamina and wire strands are welded to the nickel top platelet 15. Of course, not all the areas of contact between the platelet 15 and wire 11 are subjected to a true alloying weld, since in some areas the heat generated is suflicient only to melt the solder but not the nickel. Accordingly, the union is a combined welded and soldered joint between wire 11 and platelets 12 and 15. By this procedure and the resultant terminal assembly, it will be appreciated that a very strong and electrically sound junction is provided between platelets 15 and the respective areas 11a of the winding 11, since through the sandwich or lamination assembly, a weld junction is obtained, and the mechanical forces on platelet 15 are distributed uniformly over a large area of windings, and the windings have been embedded, Without breaking or weakening, within the laminated structure.

To complete the fabrication of a unitary resistance element With end terminals, the `strip 10, with its' winding 4 l 11 and assembled terminal structures is divided into a plurality of sections by severing the strip along the transverse center of each terminal assembly 12, 15, leaving half of a terminal assembly for one resistance unit, and the other half of each terminal assembly for the adjacent resistance unit. The resultant resistance unit is shown in FIG. 4.

The purpose of forming the present terminal assemblies is to facilitate the attachment of wire leads or the like to the resultant resistance unit. If it were attempted to solder or Weld a lead to the surface of platelet 15 over the solder-weld area, it is apparent that the solder-weld laminated structure so carefully constructed may be destroyed or damaged. Accordingly, platelets 15 are provided with extending tabs 15a which project beyond the areas of the solder-welds and are removed from the resistance wire. Leads may be readily soldered or welded to the projecting tabs 15a without in any way disturbing the laminated structure of the terminal assembly.

From the foregoing specific embodiment, it is seen that the present invention provides a wire wound resistance unit with connection terminals. The connection terminals are laminated structures including platelets located to either side of the strands of the resistance wire in the terminal area, and sandwiching these strands therebetween. Further said two platelets are fused together, and said wire strands are embedded in the fused metal forming the junction between the two platelets. As a result, there is provided a strong and electrically sound terminal structure for the resistance unit, in which the strength of the terminal is derived from the inclusion of numerous strands of the resistance wire and the grasping of these strands between the two fused platelets and the embedment of the strands in the fused material.

As previously mentioned, although the described speciiicdembodiment of the invention relates to rectilinear resistance units, it is obviously equally applicable to curved units. Also, the terminals may certainly be provided as intermediate taps, as Well as end structures; other resistance wires than Nichrome may be employed; other specific metals may be employed for the platelets; and similarly, numerous other modifications and variations of the described embodiment will become apparent to those skilled in the art. Accordingly, such variations and modications as are embraced by the spirit and scope of the appended claims, are contemplated as within the purview of the present invention.

What is claimed is:

1. A connection terminal assembly for a wire wound resistor of the type having an insulating base and a Wire resistance element wound thereon, comprising a rst plate element interposed between a plurality of turns of said wound resistance element and said base, Said rst plate element being in surface contact with said insulating base and including a body of high temperature solder in contact with said wire resistance element, a second plate element overlying said plurality of turns of said resistance element sandwiching said plurality of turns between said two plate elements, said second plate element including a material in direct contact with said resistance element which is similar in composition and melting point to said resistance element, said elements being welded together and said wire resistance element being embedded in said solder of said rst plate element.

2. A connection terminal assembly as set forth in claim 1, wherein said solder is silver solder.

3. A connection terminal assembly as set forth in claim l, wherein said second plate element has a projecting tab for facilitating the making of an electrical connection t0 the terminal assembly.

4. A connection terminal assembly as set forth in claim l, wherein said rst plate element is a laminated structure having a base material in surface contact with said insulating base and said solder is carried thereon, and said second plate lelement is composed entirely of a material similar in composition and melting point to said resistance element.

5. A connection terminal assembly as set forth in claim 4, wherein the base material of said rst plate element is brass, said material of said second element is nickel, said resistance element is Nichrome wire, and said solder is silver solder.

6. A method of forming a resistor with a connection terminal thereon, comprising winding a wire resistance element on a base of electrically insulating material, nterposing a lrst plate element between a plurality of turns of said wound resistance element and said base, said rst plate element being in surface contact with said insulating base and including a body of high temperature solder in contact with said wire resistance element, superposing a second plate element over said plurality of turns of said resistance element sandwiching said plurality of turns between said two plate elements, said second plate element including a material in direct contact with said resistance element which is similar in composition and melting point to said resistance element, and welding said elements to gether and embedding said wire resistance element in said solder of said first plate element by applying a pair of resistance welding electrodes on the second plate element.

7. A method of forming a resistor with a connection terminal thereon as set forth in claim 6, wherein said first plate element is a laminated structure having a base material in surface contact with said insulating base and said solder is carried thereon, and said second plate element is composed entirely of a material similar in composition and melting point to said resistance element.

8. A method as set forth in claim 6, wherein said solder is silver solder.

References Cited in the le of this patent UNITED STATES PATENTS 

